Quake :

Activate the console window and write :

God - God mode

Noclip - Walk through walls

Fly - Flymode

Notarget - The enemies will not fire at you

Impulse9 - Don't remember

Kill - Commit suicide

Doom :

Just type :

iddqd - God mode

idkfa - all weapons

idspispopd - walk through walls

idbehold + s,v,i,r,l

S = Berserk

V = Imortality

I = Invisibility

R = Biosuit

L = Lights on

Shadow Warriors :

First press t then type :

swgreed - Ammo, weapons etc

swchan - invisibility

swgimme - don't know really

Duke Nukem 3D :

Just type :

dnkroz - immortal

dnstuff - happy ammo

dnitems - keys,weapons etc

dnscotty+levelnumber - choose level